Tetrahydropyrimidine-2-one compounds are known to be useful in pharmaceutical preparations. For example, U.S. Pat. No. 3,821,221 discloses a number of such compounds, and their effect as central nervous system stimulants or depressants. The compounds are said to be of value for therapeutic applications as potential psychotropic drugs.
As a result of pharmacological research into these tetrahydropyrimidine-2-one derivatives, it was discovered that icilin (also known as AG-3-5, chemical name 1-[2-hydroxyphenyl]-4-[2-nitrophenyl]-1,2,3,6-tetrahydropyrimidine-2-one) produced sensations of coldness when in contact with mucous membranes (nostrils, lips and eyelids) of the researchers, and also when ingested (see Wei et al, J. Pharm. Pharmacol. 1983, 35:110–112).
A known compound for producing a sensation of cold is menthol (2-isopropyl-5-methyl-cyclohexanol), which has been extensively applied as an additive in, for example, foodstuffs and oral hygiene products. It is used primarily because it elicits a sensation of coolness in the mouth, and because it has a pleasing mint flavour and odour. The cooling effect of menthol is due to the action of menthol on the nerve endings of the human body which detect hot and cold stimuli. In particular, menthol is believed to activate cold receptors on nerve endings. However, the use of menthol is limited by its strong minty smell and relative volatility.
It was found that icilin was capable of producing the same cooling effect as menthol. Icilin has a number of advantages over menthol, for example it is more potent, and has a lower acute toxicity, due to its lack of anaesthetic properties. Icilin was considered to be a particularly useful compound for pharmacological applications because it lacks the flavour and odour of menthol and is not readily absorbed through the skin. However, icilin has not been disclosed as a replacement for menthol for non-pharmaceutical applications.
It has now been found that to formulate cooling compounds in the form of microemulsions to be incorporated in food products or beverage produces a lingering cooling effect and sometimes also, a smooth mouthfeel. “Mouthfeel” is used herein to describe the mingled experience derived from the sensations of the oral cavity during consumption of a food or beverage. It involves a product's entire physical and chemical interaction in the mouth relating to the density, viscosity, surface tension, mouthcoating, astringency, smoothness, lubricity and such like.
U.S. Pat. No. 4,835,002 describes microemulsions of essential oils which also contain certain alcohols, surfactant components and water and which are used in foods and beverages.
U.S. Pat. No. 5,045,337 describes edible water-in-oil microemulsions which are thermodynamically stable, clear and homogeneous and which contain a polar solvent, a specific polyglycerol mono diester and a lipid which may be a longer chain triglyceride.
U.S. Pat. No. 5,283,056 describes stable transparent oil-in-water microemulsion concentrates containing water, one or more hydrophobic flavour or fragrance oils and one or more surfactant components.
Australian Patent Application 58449/98 describes oil-in-water microemulsions contain triglycerol monofatty acid esters as emulsifier and a lipophilic substance which is a carotenoid, vitamin or polyunsaturated fatty acid as the internal phase of the microemulsion.
International Patent Application 99/62357 describes water-in-oil microemulsions containing flavour precursors which are activated during consumption to provide the flavour to the consumer.
European Patent Application 829206 describes edible microemulsions which contain an oil which may be a medium chain triglyceride comprising C6 to C18 fatty acids which are used to coat foodstuffs so that they brown and crisp when subjected to microwave radiation.
It is also known to make microemulsions using non-food grade oils. A method for 5 making such microemulsions is described in an article by Neelima J. Kale and Loyd V. Allen, Jr., International Journal of Pharmaceutics, 57 (1989) 87–93 entitled “Studies on microemulsions using Brij 96 as surfactant component and glycerin, ethylene glycol and propylene glycol as co-surfactant components”. This product contains light mineral oil (i.e. paraffin) which is not suitable for use in foods.